Two-layer Soil Model Research Articles

Estimation of Soil Electrical Properties in a Multilayer Earth Model with Boundary Element Formulation

This paper presents an efficient model for estimation of soil electric resistivity with depth and layer thickness in a multilayer earth structure. This model is the improvement of conventional two‐layer earth model including Wenner resistivity formulations with boundary conditions. Two‐layer soil model shows the limitations in specific soil characterizations of different layers with the interrelationships between soil apparent electrical resistivity (ρ) and several soil physical or chemical properties. In the multilayer soil model, the soil resistivity and electric potential at any points in multilayer anisotropic soil medium are expressed according to the variation of electric field intensity for geotechnical investigations. For most soils with varying layers, multilayer soil resistivity profile is therefore more suitable to get soil type, bulk density of compacted soil and to detect anomalous materials in soil. A boundary element formulation is implemented to show the multilayer soil model with boundary conditions in soil resistivity estimations. Numerical results of soil resistivity ratio and potential differences for different layers are presented to illustrate the application, accuracy, and efficiency of the proposed model. The nobility of the research is obtaining multilayer soil characterizations through soil electric properties in near surface soil profile.

Mapping Depth to Argillic Soil Horizons Using Apparent Electrical Conductivity

Maps of apparent electrical conductivity [Formula: see text] of the soil profile are widely used in precision agriculture. A number of [Formula: see text] sensors are commercially available, each with a unique response function (i.e., the relative contribution of soil at each depth to the integrated [Formula: see text] reading). Our past research estimated depth to an argillic horizon (i.e., topsoil depth, TD) on claypan soils by fitting empirical equations to [Formula: see text] sensor data. The objective of this research was to determine if TD estimates could be improved by combining data from multiple [Formula: see text] sensors and by solving for TD by inverting a two-layer soil model incorporating instrument response functions. Data were obtained with three sensors having five different [Formula: see text] depth-response functions (Veris 3150 (mention of trade names or commercial products is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the US Department of Agriculture or the University of Florida), Geonics EM38 vertical dipole mode, and DUALEM-2S) on two Missouri claypan-soil fields. Soil cores obtained in each field provided measured TD data for calibration and validation. Using a numerical optimization approach, response-function models were developed for [Formula: see text] variables individually and in combination. Similarly, linear regression was applied to single and multiple variables. Root mean square error of validation [Formula: see text] of single-variable TD estimates was [Formula: see text], with better results for those variables with moderately deep [Formula: see text] response functions. Results from the model-based approach were very similar to those obtained by regressing TD on [Formula: see text]. The best calibrations using multiple variables in model inversion or regression were somewhat better than those using single variables, with [Formula: see text] of [Formula: see text] and [Formula: see text], respectively. For all approaches, highest TD errors were localized to one area of one field, possibly because soils in this area violated the model assumption of spatially homogeneous soil layer conductivity. Although these calibrations are sufficiently accurate to be useful in TD mapping, a model solution allowing layer conductivities to vary spatially should be investigated for possible improvements.

Limiting GPR in a two-layer soil model via genetic algorithms

In this paper, an optimum grounding grid that provides the conditions of GPR< E touch and minimum cost in the structures of two-layer soil model is designed and the length of total conductor and the quantity of ground rod are calculated via Genetic Algorithms (GA). A new approach is presented for the calculation of total conductor length. At the same time, the subject regarding in which layer the ground conductors and rods that form the grounding grid in a substation are to be placed in two-layer soil is analysed using GA. With this as the goal, the depth of optimum grid burial is determined. Our study is compared with the design study for a two-layer soil model in the literature. As a result, the high performance of optimum grid design that is achieved using GA is emphasized by varied applications.

Investigations of Two-Layer Earth Parameters at Low Voltage: Measurements and Calculations

Problem statement: The two-layer soil model at low magnitude voltage is assumed to be accurate for the measurement and calculation of the earth resistance of a combined grid-multiple rods electrode. The aim of this study is to measure and calculate the earth resistance of a combined grid- multiple rods electrode buried in a two-layer soil and to confirm the simplicity and accuracy of the used formula. Approach: Soil resistivity was measured using Wenner four po int method. Advanced earth resistivity measurement interpretation techni ques which include graphical curve matching based on master curves and an advanced computer program based on a genetic algorithm are used in this study. Results: Based on the resistivity data, the earth resistanc e value was calculated using the formulas obtained from the literature. Measurements of the earth resistance of the earthing system were also conducted using the fall of potential met hod. Conclusion/Recommendations: A very good agreement was obtained between the measured and calculated earth resistance values. This research is the first time ever conducted where the measured ea rth resistance values are compared directly with the calculated earth values.

Correction of resistance measurement of large grounding systems

A method for the systematic error correction which appears at the grounding system resistance measurement by means of an auxiliary current electrode, when the distance between the edge of the grounding system and the center of the current electrode is shorter than the minimum distance required by the standard, is presented in this paper. The systematic error increases with the ratio of the size of the grounding system to its distance from the current electrode. The method for the correction of the systematic error is based on modelling of grounding system and current electrode by hemispheres in uniform and two-layer soil model. The calculation results are presented by diagrams, which are useful for the evaluation of systematic error.

Visualization technique for liquefaction process in chamber experiments by using electrical resistivity monitoring

A new technique for real-time visualization of soil liquefaction by using electrical resistivity monitoring is presented. The technique utilizes the relationship between porosity and electric resistivity of soil, which was defined first by Archie in 1942. With the technique, the change of density (including relative density) of saturated soil with respect to depth can be monitored in real-time before, during, and after the dynamic excitation applied. A series of small chamber experiments on single-layer, two-layer, and five-layer soil models were performed and soil liquefaction was monitored in each case. The experiments show that the technique can be used successfully to visualize the change in density of saturated soil throughout the process of pore pressure build-up, liquefaction, and post-liquefaction during shaking experiments. The effects of sand properties, stratification, duration and magnitude of dynamic excitation, and application of successive dynamic excitations on the liquefaction and post-liquefaction behavior of soils are discussed. The technique is particularly promising for large-scale chamber experiments where it can be used to control and monitor relatively large soil mass in addition to determine its state of liquefaction during and after the shaking.

The Modified Palmer Drought Severity Index Based on the NCEP North American Regional Reanalysis

Abstract A 32-km high-resolution modified Palmer drought severity index (MPDSI) based on the National Centers for Environmental Prediction (NCEP) North American Regional Reanalysis (RR) from 1979 to 2004 is presented. The assumptions of Palmer, such as the water balance equation, the difference between observed precipitation and the climatologically expected precipitation over the maximum conditions, and the changes of the index as a function of the current index, are preserved. Many deficiencies of the original PDSI are eliminated by taking fields directly from the RR or by making better estimates. For example, fields such as potential evapotranspiration, evaporation, runoff, total soil moisture, and soil moisture change in a given month are obtained directly from the RR. The potential recharge is defined as the total soil moisture needed to reach the maximum total soil moisture at each grid point for each calendar month. The potential precipitation is defined as the maximum precipitation at each grid point for a given calendar month. The underground volumetric soil moisture includes both frozen and liquid form. Therefore, the contribution of snowmelt is taken into account inexplicitly. The questionable assumptions of two-layer soil model and the available soil moisture capacity are no longer needed. Overall, the MPDSI, when averaged over a large area and long time, often resembles the traditional PDSI based on the Palmer formula and the climate-division data. The MPDSI obeys Gaussian distribution, and so it can also be used to assess the potential for floods. Together with a consistent suite of soil moisture, surface energy, and atmospheric terms from the RR, the MPDSI can be used to monitor and diagnose drought and floods.

Soil Water Content on Mars as Estimated from Neutron Measurements by the HEND Instrument Onboard the 2001 Mars Odyssey Spacecraft

We present the results of 20 months of observations of Mars by the Russian HEND instrument onboard the NASA 2001 Mars Odyssey spacecraft. We show that there are two extended subpolar regions with a soil water content of several tens of percent in the northern and southern hemispheres of Mars. The southern subpolar region is well described by a two-layer model, according to which a soil with a water content of up to 55% by mass lies under a relatively dry soil with a water mass fraction of 2% and a thickness of 15–20 g/cm2. The distribution of water in Martian regolith northern subpolar region is in good agreement with the homogeneous model and does not require invoking the more complex two-layer soil model. The water-ice content in the subsurface layer of the northern subpolar region reaches 53 % by mass. We show that there are two regions with a relatively high water content near the Martian equator. These are Arabia Terra and the Medusae Fossae formation region southwest of Olympus Mons. In these regions, a lower layer with 9–10% of water by mass may underlie the upper layer of relatively dry material ∼30 g/cm2 in thickness. The “moistest” spot near the equator is at about 30° E and 10° N. Its lower-layer soil may contain more than 16% of water by mass.

Soil Resistivity Measurement: Two-Layer Model, Proposed Revisions to IEEE Standard 80-2000 and IEEE Standard 81-1983

Some simplification to Sunde's procedure, shown in IEEE Standard 80-2000 13.4.2.2, two-layer soil model by graphical method is proposed, with the introduction of a modified Sunde's curve. Comments on Appendix B of IEEE Standard 81-1983 are offered to correct some formula and improve concepts.

A numerical formulation for grounding analysis in stratified soils

The design of safe grounding systems in electrical installations is essential to assure the security of the persons, the protection of the equipment and the continuity of the power supply. In order to achieve these goals, it is necessary to compute the equivalent electrical resistance of the system and the potential distribution on the earth surface when a fault condition occurs. In this paper we present a formulation for the analysis of grounding systems embedded in stratified soils, on the basis of the boundary element method (BEM). Suitable arrangements of the final discretized equations allow us to use the highly efficient analytical integration techniques derived by the authors for grounding systems buried in uniform soils. The feasibility of this approach is demonstrated by applying the BEM formulation to the analysis of a real grounding system with a two-layer soil model.

2002 Summer Meeting Preview

Some simplification to Sunde's procedure, shown in IEEE Standard 80-2000 13.4.2.2, two-layer soil model by graphical method is proposed, with the introduction of a modified Sunde's curve. Comments on Appendix B of IEEE Standard 81-1983 are offered to correct some formula and improve concepts.

A Numerical Formulation for Grounding Analysis in Stratified Soils

The design of safe grounding systems in electrical installations is essential to assure the security of the persons, the protection of the equipment and the continuity of the power supply. In order to achieve these goals, it is necessary to compute the equivalent electrical resistance of the system and the potential distribution on the earth surface when a fault condition occurs. In this paper we present a formulation for the analysis of grounding systems embedded in stratified soils, on the basis of the boundary element method (BEM). Suitable arrangements of the final discretized equations allow us to use the highly efficient analytical integration techniques derived by the authors for grounding systems buried in uniform soils. The feasibility of this approach is demonstrated by applying the BEM formulation to the analysis of a real grounding system with a two-layer soil model.

A comparative study of two land surface schemes in regional climate integrations over South America

[1] A case study comparison using two different land surface models in the Regional Spectral Model (RSM) is presented here. This comparison is motivated from recent studies that show a significant impact of land surface processes on the predictability of precipitation at seasonal to interannual scales. We find in this comparative study that coupled land-atmosphere interactions using the simplified simple biosphere (SSiB) scheme and a two-layer soil (control) model with uniform vegetation fraction yield mixed results. It does not clearly indicate the superiority of one scheme over the other for this particular case. The warmer mean surface temperatures simulated by RSM-SSiB improve the seasonal (January-February-March [JFM]) simulation over Amazon River Basin, along Brazilian Highlands, over Bolivian Plateau, and over central America relative to the control model. However, the RSM-SSiB runs exhibit a strong warm bias in the surface temperature over the subtropics of South America. The mean JFM onshore easterly flow from the tropical Atlantic Ocean and the low-level jet are relatively stronger in the SSiB model. The mean JFM precipitation from the RSM-SSiB model shows an improvement over Guianan Highlands, Venezeulan Llanos, and Amazon River Basin. However, the mean JFM precipitation of the control model over the Caribbean Sea, Central America, and equatorial eastern Pacific Ocean is better than the SSiB model.

Assessment of the Land Surface and Boundary Layer Models in Two Operational Versions of the NCEP Eta Model Using FIFE Data

Data from the 1987 summer FIFE experiment for four pairs of days are compared with corresponding 48-h forecasts from two different versions of the Eta Model, both initialized from the NCEP–NCAR (National Centers for Environmental Prediction–National Center for Atmospheric Research) global reanalysis. One used the late 1995 operational Eta Model physics, the second, with a new soil and land surface scheme and revisions to the surface layer and boundary layer schemes, used the physics package that became operational on 31 January 1996. Improvements in the land surface parameterization and its interaction with the atmosphere are one key to improved summer precipitation forecasts. The new soil thermal model is an improvement over the earlier slab soil model, although the new skin temperature generally now has too large a diurnal cycle (whereas the old surface temperature had too small a diurnal cycle) and is more sensitive to net radiation errors. The nighttime temperature minima are often too low, because of a model underestimate of the downwelling radiation, despite improvements in the coupling of the surface and boundary layer at night. The daytime incoming solar radiation has a substantial high bias in both models, because of some coding errors (which have now been corrected), insufficient atmospheric shortwave absorption, and underestimates of cloud. The authors explore evaporation before and after a midsummer heavy rain event with the two models. The late 1995 operational model uses a soil moisture bucket physics, with a specified annual-mean fixed field soil moisture climatology, so the surface evaporation responds primarily to the atmospheric forcing. While the surface fluxes in the new model show this strong rain event more dramatically, because its soil moisture comes from the global reanalysis rather than climatology, there remain problems with soil moisture initialization. It appears that a fully continuous Eta data assimilation system (which is under development), likely with more than two soil layers and assimilation of observed hourly precipitation, will be needed to get an adequate soil moisture initialization. Evaporation in the new two-layer soil model falls too much from forecast day 1 to day 2, as the first shallow 10-cm layer dries out (as it also does in the 1995 model with the bucket physics). This appears to be related to the specified low vegetation fraction and the bare soil evaporation model. Although the new boundary layer scheme is better coupled to the surface at night, both versions underestimate entrainment at the top of the mixed layer. The improvement in the surface evaporation resulting from using a climatological green vegetation fraction (derived from satellite data) and a revised bare soil evaporation formulation are shown. These changes were incorporated in a model physics revision in February 1997. An encouraging result from one case study, when it rained in the model, shows that the interaction between the surface, boundary layer, and convection schemes during precipitation is satisfactory, although the model underestimates the impact of cloud cover on the incoming solar radiation.

96/06114 Two-layer soil model for power station grounding system calculation considering multilayer soil stratification

96/06114 Two-layer soil model for power station grounding system calculation considering multilayer soil stratification

Two-layer soil model for power station grounding system calculation considering multilayer soil stratification

Two-layer soil models are commonly used for the design of power station grounding systems. Calculations based on such models usually give correct values for the gound resistance of grounding electrodes and for the step and touch potentials on the surface of the ground. However, the two-layer model of the actual soil can give absurd results if incorrect values of the layer resistivities and the first layer thickness are used. Conventional methods of fitting the apparent resistivity curve in order to obtain those parameters do not give good results in all cases. This may happen when the soil has a multilayer structure and certain combinations of different layer resistivities exist. In this paper, an analysis of critical cases was carried out, and a novel calculation method for the parameters of the two-layer soil model is presented. The proposed method is valid for an arbitrary number of soil layers with arbitrary values of resistivity, and it is applicable to simple as well as complex grounding systems. The numerical results of this method were compared with electromagnetic field calculations carried out with a computer program based on the finite-element method. The proposed model permits the use of computer programs based on the method of images and a two-layer soil model for the design of station grounding systems buried in horizontal multilayer soil stratifications. Thus, the risk of serious calculation errors that may appear by application of the apparent resistivity fitting method can be avoided.

Estimation of two layer soil parameters using finite Wenner resistivity expressions

An efficient computer-based method for evaluation of an optimal two-layer soil model from Wenner four probe test data is presented. The method is based on recently developed finite expressions for computation of potential in two-layer soil. The coefficients of the formulae have been readjusted to suit the present application. A computer program SPEF (Soil Parameters Estimation using Finite Expressions) based on the proposed method is described. The accuracy and efficiency of the method are established by determining two-layer soil models for several sets of apparent resistivity data. SPEF is shown to be much more efficient and convenient than programs based on infinite series expressions.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Sensitivity study and validation of a land surface parameterization using the HAPEX-MOBILHY data set

A simple parameterization of land surface processes, amenable to the structure of a two-layer soil model, including a representation of the vegetation, has been designed for use in meteorological models. Prior to implementation in a mesoscale model, it is necessary to check the components and to verify the good working order of the parameterization as a whole. The aims of this paper then are: (i) evaluation and a sensitivity study of the various components of the model, specifying the needed accuracy for the parameters; (ii) micrometeorological validation of the model against the HAPEX-MOBILHY data set.

The ground grid design of the Barra do Peixe substation using a two-layer soil model

A computer program (originally developed for electric field calculations) has been successfully used for the ground grid design of a substation (Barra do Peixe) to be build in the northwest part of Brazil. An additional computer program for soil modeling, which determines the best two-layer fit from multilayered earth, is presented. The results of earth resistivity field measurements and their analytical interpretation are presented. The resulting grid design, layout, and safety characteristics are reported.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Evaluation of the voltage distribution around toroidal HVDC ground electrodes in n-layer soils

A method for evaluating the electrical voltage and the current density in the immediate vicinity of a toroidal grounding installation of DC substations is presented. In order to guarantee that voltage rises generated during operation are within safe limits and considering the need to reduce the dimensions and costs of grounding installations, it is necessary in certain cases to improve the soil model in the area affected by the electrode. Electrostatic theory is used to generate a voltage solution for the Laplacian equation for n-layer soil models, and computed voltage profiles are shown in the vicinity of an actual electrode. Theoretical results for a two-layer soil model are also presented, along with measurements carried out on a reduced-scale electrode.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>